The present invention relates to optical fibers suitable for the Raman amplification, and optical amplifier and optical transmission system, which involve the optical fiber.
Wavelength Division Multiplexing (WDM) transmission, which enables high-capacity transmission by transmitting plural optical signals through a single optical fiber, has been in widespread use in the field of telecommunications.
The Er-Doped Fiber Amplifier (EDFA) is developed and used for the relay points in a WDM transmission system. In addition, optical fibers doped with other rare earth elements than erbium are under study for the optical-amplification. The amplification with rare-earth doped optical fibers needs no transformation from optical signals to electric signals at every wavelength in the preceding relay points, thereby giving an impetus to the spread of WDM transmission.
Meanwhile, Raman amplifiers with the Raman amplification are arresting attention as a novel optical amplifier. For instance, Distributed Raman Amplification is eagerly investigated to improve the transmission characteristics by applying to an optical transmission line as a Raman amplification component.
Moreover, as authoritative reports in Tus5, OFC'99 and OtuB6, Optical Amplifiers and Their Applications 2001 presented, Discrete Raman Amplification is under vigorous study, using a DCF (dispersion compensating fiber) for single-mode fiber (SMF). The study is designed to attach the DCF with a function of optical amplification.
The Raman amplification is a technique to utilize induced emission in Raman scattering for the amplification of optical signals, by a frequency of 13.2 T Hz lower than that of the pump light source (roughly on the 100-110 nm longer wavelength side ). The amplification efficiency depends largely on the non-linearity of an optical fiber. In general, the higher in non-linearity of optical fiber, the more in Raman amplification efficiency.
The above DCF was designed in some cases for a purpose to compensate the positive dispersion arising in a single-mode optical fiber or a non-zero dispersion-shifted fiber (NZ-DSF) with the zero-dispersion shifted to a wavelength slightly off 1.55 μm, as short length as possible. Therefore, the DCF has a negative dispersion of no more than −70 ps/nm/km at a wavelength of 1.55 μm (hereinafter referred to as “at 1.55 μm”), and an effective area of 15-20 μm2 at 1.55 μm.
The DCF, which has a small effective area and a high non-linearity, is extremely suitable for a Raman amplification component.